Processing method and apparatus for removing oxide film

ABSTRACT

Disclosed are a processing method and apparatus for removing a native oxide film from the surface of a subject to be treated. In this method and apparatus, gas generated from N 2 , H 2  and NF 3  gases is reacted with the surface of the subject to degenerate the native oxide film into a reactive film. If the subject is heated to a given temperature, the reactive film is sublimated and thus the native oxide film is removed. Plasma is generated from the N 2  and H 2  gases and then activated to form an activated gas species. The NF 3  gas is added to the activated gas species to generate an activated gas of these three gases. In the step of forming the reactive film, the subject is cooled to not higher than a predetermined temperature by a cooling means. In the step of sublimating the reactive film, the subject is lifted up to a predetermined heating position. Also disclosed is a cluster system which includes the treatment apparatus of the present invention and other apparatuses and which is capable of carrying a subject to be treated in an unreactive atmosphere.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation-in-Part application of U.S. patent application Ser. No. 09/086,574, filed May 29, 1998, now abandoned the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a processing method and apparatus for removing an oxide film formed on a surface of an object and a contaminant to be treated mainly such as a semiconductor wafer, particularly, to a processing method and apparatus for removing a thin oxide film such as a native oxide film formed on a surface of the object such as a semiconductor wafer, and more particularly, to a processing method and apparatus for removing a native oxide film formed in a bottom portion of a fine hole formed on a surface of the object such as a semiconductor wafer.

The present invention is directed mainly to a technique of removing an oxide film formed on a surface of an object such as a semiconductor wafer, particularly, to a technique for removing a native oxide film. Although, the object to be processed-in-the present invention is not limited to a semiconductor wafer, the related art in relation to a technique for removing a native oxide film formed in a bottom portion of a fine hole formed on a surface region of a semiconductor wafer in order to specifically describing the related art is described.

As widely known to the art, processes to form a film on a semiconductor wafer used as a substrate and etching treatments to selectively etch the resultant film in a predetermined pattern are repeatedly carried out in the manufacture of a semiconductor integrated circuit on the semiconductor wafer. During these processes, the substrate is transferred among various processing devices. During the transfer, the substrate is exposed to the air atmosphere, with the result that the oxygen and water within the air atmosphere unavoidably cause a native oxide film to be formed on a surface of the substrate. Formation of the native oxide film causes the properties such as electrical properties of the film on a surface of the substrate to be deteriorated. Where deterioration of the film properties is undesirable in the process for forming a film on the substrate or in the etching process of the film, it is necessary to remove the native oxide film formed on the substrate.

A wet etching is one of the conventional techniques for removing the native oxide film. In the wet etching, the semiconductor substrate (wafer) having a native oxide film formed thereon is immersed in a washing liquid for removing the native oxide film. It should be noted that the line width of a wiring and the diameter of a contact hole formed in the semiconductor wafer are diminished with increase in the scale of integration and miniaturization of the semiconductor integrated circuit. For example, the diameter of the contact hole is 0.2 to 0.3 μm or less (e.g., 0.12 μm). What should be noted is that, since the contact hole has a very small diameter, the washing liquid is unlikely to enter sufficiently the contact hole. Also, the washing liquid once entering the contact hole is not likely to be easily expelled from within the contact hole because of the surface tension of the washing liquid. Under the circumstances, it is difficult for the washing liquid to remove sufficiently a native oxide film formed in a bottom portion of the contact hole.

Where the substrate is subjected to a wet washing, the wall of the contact hole is also etched together with the native oxide film. It should be noted in this connection that the contact hole extends through a plurality of layers formed on the substrate, with the result that the wall of the contact hole consists of a plurality of these layers. What should be noted is that these plural layers differ from each other in the etching rate when subjected to etching with the wet washing liquid. It follows that the surface of the contact hole is rendered irregular after the etching with the wet washing liquid. FIGS. 6A and 6B show in detail the situation. Specifically, FIG. 6A shows that a contact hole 2 for achieving an electrical contact with a drain or source region is formed in a surface region of a silicon substrate W. The contact hole 2, which has a diameter of about 0.2 to 0.3 μm, extends through three layers consisting of a SiO₂ λ1 layer 4 formed by thermal oxidation, a phosphorus-doped glass (SiO₂) λ2 layer 6 formed by a spin-coating method, and a silica glass (SiO₂) λ3 layer 8, as shown in the drawing. A native oxide film 10 is formed at the bottom of the contact hole 2. These SiO₂ layers 4, 6 and 8 slightly differ from each other in the etching rate when washed with a washing liquid. It follows that, if the native oxide film 10 is removed by the wet etching, the wall surface of the contact hole 2 is caused to be irregular by the difference in the etching rate noted above, as shown in FIG. 6B. In addition, the washing liquid tends to enter the boundary regions between the adjacent two layers, leading to an over-etching of the boundary regions.

To overcome the above-noted difficulties, it is proposed to employ a so-called dry etching method in place of the wet etching method for removing the native oxide film at the bottom of the contact hole. Japanese Patent Disclosure (Kokai) No. 2-256235 discloses a method of removing a native oxide film by utilizing a NF₃ gas (halogen gas) or NH₃ gas (basic gas). It is disclosed that the halogen gas or the basic gas noted above is introduced into a process chamber, and the native oxide film is removed by plasma formed within the process chamber. In this technique, however, required is an apparatus for exhausting these two kinds of the special gases (NF₃, NH₃) leading to a high operating cost. Japanese Patent Disclosure No. 6-338478 discloses another technique. It is disclosed that an H₂ gas and an H₂O vapor are supplied into a plasma generating section for activation of these gas and vapor. Then, an NF₃ gas or a gaseous mixture containing NF₃ gas is added to the activated gas and vapor for removing the native oxide film. However, since H₂O (steam) is used in this technique, a native oxide film tends to be formed in an amount larger than the amount of the removed native oxide film. As a matter of fact, a native oxide film was not sufficiently removed in the experiment conducted by the present inventor.

In order to resolve the above drawback of the conventional wet cleaning, a method of removing a native oxide film from a subject to be treated using etching gas, i.e., a so-called dry cleaning (etching) method is proposed in, for example, Jpn. Pat. Appln. KOKAI Publication Nos. 5-275392, 6-338478, and 9-106977.

FIG. 14 shows a prior art dry etching apparatus for dry-etching an SiO₂ film by the dry cleaning method as disclosed in the above No. 5-275392 Publication. The dry cleaning method for eliminating a native oxide film from a subject to be treated, will now be described with reference to FIG. 14 showing the dry etching apparatus. In the apparatus shown in FIG. 14, an open/close valve 450 is closed to cut off Ar gas from an Ar-gas source 454. Open/close valves 436 and 438 are opened to supply NF₃ gas and H₂ gas from an NF₃-gas source 444 and an H₂-gas source 446 to a pipe 432 by controlling their flow rates by means of flow-rate controllers (MFC) 440 and 442. In the pipe 432, both the NF₃ gas and H₂ gas are mixed at a mixing ratio of 1:2 into a mixed gas having a total pressure of 0.2 Torr. A 2.45-GHz-frequency, 50-w-power microwave is supplied from a magnetron into the pipe 432 via a microwave waveguide 448, and the mixed gas thus becomes plasma therein. A fluorine active species F*, a hydrogen active species H*, and a nitrogen active species N*, which are generated by the plasma, move toward a chamber 410 within the pipe 432 and enter a buffer chamber 430 of the chamber 410. These species are then supplied downstream onto a wafer W placed on a susceptor 412 through a porous plate 428. The wafer W is cooled by a chiller which is supplied from a chiller supply unit 418 and cooled to not higher than room temperature. The active species F*, H* and N* supplied to the cooled wafer W, are adsorbed by the native oxide film on the surface of the wafer W and react to SiO into a product. This product is vaporized and exhausted from an exhaust hole 460 provided at the bottom of the chamber 410 by a vacuum pump 466.

In the foregoing prior art method of removing a native oxide film from the surface of a cooled wafer by fluorine, hydrogen and nitrogen active species F*, H* and N* generated by plasma, which is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 5-275392, NF₃ is changed into plasma and thus dissolved into fluorine and nitrogen active species F* and N*, so that no NF₃ active gas can be generated efficiently. Since, moreover, H₂ gas has difficulties in maintaining the plasma state by itself, it is difficult to secure an etching rate enough to eliminate the native oxide film.

In another method of eliminating a native oxide film by dry cleaning, which is disclosed in Jpn. Pat. Appln. KOKAI Publications Nos. 6-338478 and 9-106977, it is difficult to secure an etching rate enough to remove a native oxide film since H₂ gas is used alone.

The present invention aims at resolving the above problems or drawbacks of the prior art method of removing an oxide film such as a native oxide film. According to the present invention, in order to remove an oxide film having a thickness of 10 Å to 20 Å from the surface of a subject to be treated, H₂ gas and N₂ gas are mixed into plasma gas, and NF₃ gas (reactive gas) is added to the plasma gas during the flow of active species of the mixed gas. The subject is cooled to not higher than room temperature, and the oxide film on the subject reacts with the reactive gas to form a reactive film. After that, the subject to be treated is heated to a given temperature or higher, and the reactive film is removed from the surface of the subject.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention, which has been achieved in an attempt to solve the above-noted problems, is to provide a method and apparatus for effectively removing an oxide film formed on a surface of an object mainly such as a semiconductor wafer.

Another object is to provide a method and apparatus for effectively removing a native oxide film formed on a surface of an object mainly such as a semiconductor wafer.

Another object is to provide a processing method and apparatus, which do not require a high cost for disposing of an exhaust gas.

Another object is to provide a method and apparatus for effectively removing a native oxide film without newly forming a native oxide film.

Further, still another object of the present invention is to provide a method and apparatus for effectively removing a native oxide film formed at a bottom portion of a fine hole formed in a surface region of an object such as a semiconductor wafer.

Further, still another object of the present invention is to provide a cluster system wherein at least one metal-wiring forming chamber is provided in the above apparatus such that a subject to be treated can be carried in an unreactive atmosphere.

According to a first aspect of the present invention, there is provided a method of removing an oxide film formed on a surface of an object to be processed, comprising the steps of:

forming an activated gas from a N₂ gas, H₂ gas and NF₃ gas;

exposing a surface of the object to the activated gas to bring about a reaction between the activated gas and an oxide film formed on a surface of the object, thereby forming a reaction film; and

heating the object to a predetermined temperature so as to sublimate the reaction film.

Preferably, the oxide film to be removed by the above method is a native oxide film.

Preferably, in the above method, the formation of the activated gas from the N₂ gas, H₂ gas and NF₃ gas comprises the steps of forming a plasma of a mixed gas consisting of the N₂ gas and H₂ gas and also forming active species, and supplying the NF₃ gas into the active species so as to form activated gases of the N₂ gas, H₂ gas and NF₃ gas.

Preferably, in the above method, the formation of a plasma of a mixed gas consisting of the N₂ gas and H₂ gas is carried out in a quartz-made plasma generating section by introducing a microwave into a mixed gas of the N₂ gas and H₂ gas supplied to the plasma generating section.

Preferably, in the above method, the predetermined temperature is 100° C. or higher.

Preferably, in the above method, the processing using an activated gas is executed in room temperature and, after a supply of the activated gas is stopped, the reaction film is sublimated by heating the object to a predetermined temperature.

According to a second aspect of the present invention, there is provided a processing apparatus for removing an oxide film, comprising:

a susceptor on which an object to be processed is disposed;

a process chamber housing said susceptor; and

a mechanism for removing oxide films formed on a surface of the object;

wherein said mechanism for removing the oxide films include:

an activated gas forming device for forming an activated gas from N₂ gas, H₂ gas and NF₃ gas;

an introducing device for introducing the activated gas formed in said activated gas forming device onto a surface of the object disposed on said susceptor arranged within said process chamber; and

a heating device for heating the object to temperatures at which reaction films resulting from reaction between the oxide films formed on the surface of the object and the activated gas introduced into the process chamber are sublimated.

Preferably, in the above processing apparatus, said activated gas forming device includes:

a plasma generating device for converting the supplied gas into plasma;

a gas supply device for supplying N₂ gas and H₂ gas into said plasma generating device;

an activated species forming device for converting the plasma generated from the plasma generating device into activated species; and

an activated gas forming device for supplying an NF₃ gas into the activated species of N₂ gas and H₂ gas formed in said activated species forming device so as to form activated gases of N₂ gas H₂ gas and NF₃ gas.

Preferably, in the above processing apparatus, the oxide film formed on the surface of the object is a native oxide film formed by the reaction with the air atmosphere during the predetermined process steps applied to the object.

Preferably, in the above processing apparatus, said activated gas forming device includes:

a plasma generating device for converting the supplied gas into plasma;

a gas supply device for supplying N₂ gas and H₂ gas into said plasma generating device;

an activated species forming device for converting the plasma generated from the plasma generating device into activated species; and

an activated gas forming device for supplying an NF₃ gas into the activated species of N₂ gas and H₂ gas formed in said activated species forming device so as to form activated gases of N₂ gas H₂ gas and NF₃ gas, and

wherein the heating device for heating the object is for heating the object to temperatures at which the native oxide films formed on the surface of the object react with the activated gas introduced into the process chamber and the resultant reaction films are sublimated.

Preferably, in the above processing apparatus, said plasma generating device is equipped with a mechanism for converting a gas into a plasma by utilizing a microwave.

Preferably, in the above processing apparatus, said activated gas forming device includes:

a pipe made of microwave transmitting material; and

a supply section of a microwave and a supply section of an N₂ gas and H₂ gas formed at the inlet port of said pipe.

Preferably, in the above processing apparatus, an introducing mechanism for introducing said activated gas onto the surface of the object disposed on the susceptor arranged in the process chamber includes a guide arranged at the outlet port of said activated species forming device for guiding the activated gases of the N₂ gas, H₂ gas and NF₃ gas onto the surface of the object.

Preferably, in the above processing apparatus, those walls of said activated gas forming device, said introducing mechanism and said process chamber which are brought into contact with said activated gas are formed of an electrically insulating material.

Preferably, in the above processing apparatus, said heating device heats said susceptor so as to elevate the temperature of the object disposed on the susceptor to temperatures at which said reaction films are sublimated.

According to a third aspect of the present invention, there is provided a surface treatment method comprising the steps of:

carrying a subject to be treated, which has an oxide on a surface thereof, into a treatment vessel;

evacuating the treatment vessel to produce a vacuum;

introducing gas containing N and H gases into a plasma generation section, generating plasma from the gas, and activating the plasma to form an activated gas species of N and H gases;

causing the activated gas species to flow toward the subject and adding an NF₃ gas to the activated gas species to generate an activated gas of NF₃ gas;

cooling the subject to not higher than a predetermined temperature; and

reacting the activated gas of NF₃ gas with the oxide on the surface of the subject to degenerate the oxide into a reactive film.

Preferably, in the above surface treatment method, the gas containing N and H gases is a mixture gas of N₂ and H₂ gases, and the method further comprises the steps of:

stopping supply of N₂, H₂ and NF₃ gases into the treatment vessel and heating the subject to a predetermined temperature to sublimate the reactive film, after the step of degenerating the oxide into the reactive film; and

stopping evacuation of the treatment vessel and taking the subject, from which an oxide film is removed, out of the treatment vessel.

According to a fourth aspect of the present invention, there is provided a surface treatment method comprising the steps of:

carrying a subject to be treated, which has an oxide on a surface thereof, into a treatment vessel;

evacuating the treatment vessel to produce a vacuum;

introducing gas containing N and H gases into a plasma generation section, generating plasma from the gas, and activating the plasma to form an activated gas species of N and H gases;

causing the activated gas species to flow toward the subject and adding an NF₃ gas to the activated gas species to generate an activated gas of NF₃ gas;

cooling the subject to not higher than a predetermined temperature; and

reacting the activated gas of NF₃ gas with the oxide on the surface of the subject to degenerate the oxide into a reactive film.

Preferably, in the above surface treatment method, the gas containing N and H gases is a mixture gas of N₂ and H₂ gases, and the method further comprises the steps of:

stopping supply of N₂, H₂ and NF₃ gases into the treatment vessel and heating the subject to a predetermined temperature to sublimate the reactive film, after the step of degenerating the oxide into the reactive film; and

stopping evacuation of the treatment vessel and taking the subject, from which an oxide film is removed, out of the treatment vessel.

Preferably, in the above surface treatment method, the predetermined temperature at which the subject is cooled, is not higher than room temperature.

Preferably, in the above surface treatment method, the predetermined temperature at which the subject is cooled, ranges from 20° C. to −20° C.

Preferably, in the above surface treatment method, the predetermined temperature at which the subject is cooled, ranges from 10° C. to −20° C.

Preferably, in the above surface treatment method, the predetermined temperature at which the reactive film is sublimated, is not lower than 100° C.

According to a fifth aspect of the present invention, there is provided a surface treatment apparatus comprising:

a plasma generation section for generating plasma from a plasma generating gas;

a treatment vessel connected to the plasma generation section and including a susceptor on which a subject to be treated is placed;

cooling means for cooling the subject placed on the susceptor to a predetermined temperature;

lifting means for lifting the subject to a heating position in the treatment vessel; and

heating means for heating the subject to a predetermined temperature in the heating position.

Preferably, the above surface treatment apparatus is an apparatus for removing a native oxide film from a surface of the subject to be treated.

Preferably, the above surface treatment apparatus further comprises:

a plasma generating gas introduction section for introducing N₂ and H₂ gases to the plasma generation section as a plasma generating gas; and

an NF₃-gas supply section for adding an NF₃ gas to an activated gas species of N₂ and H₂ gases activated by the plasma generation section and caused to flow toward the subject to be treated, and

an activated gas of NF₃ gas is generated by adding the NF₃ gas to the activate gas species, and the activated gas is reacted with a surface layer of the subject to degenerate the surface layer.

Preferably, in the above surface treatment apparatus, the predetermined temperature at which the subject placed on the susceptor is cooled, is not higher than room temperature.

Preferably, in the above surface treatment apparatus, the predetermined temperature at which the subject placed on the susceptor is cooled, ranges from 20° C. to −20° C.

Preferably, in the above surface treatment apparatus, the predetermined temperature at which the subject placed on the susceptor is cooled, ranges from 10° C. to −20° C.

Preferably, in the above surface treatment apparatus, the predetermined temperature at which the subject is heated at the heating position, is not lower than 100° C.

Preferably, in the above surface treatment apparatus, the NF₃-gas supply section includes a number of gas exhaust holes formed in an inner wall of the treatment vessel.

Preferably, in the above surface treatment apparatus, the NF₃-gas supply section includes a shower head having a number of gas exhaust holes provided in the treatment vessel.

Preferably, in the above surface treatment apparatus, the NF₃-gas supply section supplies the NF₃ gas to the activate gas species in position at least 20 cm away from an end of the plasma generation section in a direction-of the subject to be treated.

Preferably, in the above surface treatment apparatus, the heating means is heat radiation means provided above the subject to be treated.

Preferably, in the above surface treatment apparatus, the heating means is a heating lamp provided above the subject to be treated.

According to a sixth aspect of the present invention, the above surface treatment apparatus comprises a cluster system including at least one metal-wiring forming chamber, a heating chamber, and a load-lock chamber such that the subject is carried through a carrier chamber in an unreactive atmosphere.

According to a seventh aspect of the present invention, the above surface treatment apparatus comprises a cluster system including at least one metal-wiring forming chamber, a heating chamber, a cooling chamber, and a load-lock chamber such that the subject is carried through a carrier chamber in an unreactive atmosphere.

Preferably, in the above cluster system, the metal-wiring forming chamber is a chamber for forming a film of at least one of Al, Ti, TiN, Si, W, WN, Cu, Ta, TaN and SiN.

Preferably, in the above cluster system, the metal-wiring forming chamber includes means for heating the subject to a temperature of 100° C. or higher.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings are related to and constitute a part of the specification and exemplifies a preferred embodiment of the present invention, wherein:

FIG. 1 shows the construction of a processing apparatus according to one embodiment of the present invention;

FIG. 2 is a plan view showing an NF₃ gas supply section;

FIGS. 3A to 3C show the processing steps of a processing method of the present invention;

FIG. 4A is a graph showing the result of an elemental analysis on a surface of an object before processing by the method of the present invention;

FIG. 4B is a graph showing the result of an elemental analysis on a surface of an object after processing by the method of the present invention;

FIG. 5 shows a modification of the plasma forming section;

FIGS. 6A and 6B are directed to a conventional method for removing a native oxide film;

FIG. 7 is a graph showing the relationship between the contact resistance and the contact size;

FIG. 8 is a schematic block diagram of a surface treatment apparatus according to another embodiment of the present invention;

FIG. 9A is a plan view of one example of a wafer lifting mechanism which can be applied to the surface treatment apparatus shown in FIG. 8;

FIG. 9B is a side view of the wafer lifting mechanism illustrated in FIG. 8;

FIG. 10A is a plan view showing a ring-shaped shower head as an alternative to an NF₃-gas supply section (shower head) which is applicable to both the treatment apparatuses of FIGS. 1 and 8;

FIG. 10B is a plan view showing a grid-like shower head as an alternative to the NF₃-gas supply section which is applicable to both the treatment apparatuses of FIGS. 1 and 8, the grid-like shower head being viewed from a susceptor;

FIG. 11 is a flowchart of respective steps of a surface treatment method according to the present invention;

FIG. 12 is a conceptual view of a vacuum cluster system constituted by a combination of a heating unit and a wiring-forming unit when the surface treatment apparatuses shown in FIGS. 1 and 8 are each employed as a native oxide film removing apparatus;

FIG. 13 is a conceptual view of a vacuum cluster system constituted by a combination of a heating unit, a wiring-forming unit and a cooling unit when the surface treatment apparatuses shown in FIGS. 1 and 8 are each employed as a native oxide film removing apparatus;

FIG. 14 is a schematic view of a prior art etching apparatus which is applied to a method of removing a native oxide film by etching using a neutral gas species of NF₃ and H₂ gases;

FIG. 15 is a conceptual diagram showing an alternative to an NF₃-gas supply section which is applicable to the surface treatment apparatus of the present invention; and

FIG. 16 is a diagram showing a relationship between etching rate and temperature at which the wafer is cooled in the surface treatment apparatus of the present invention.

DESCRIPTION OF THE INVENTION

The present invention is directed mainly to a technique of removing an oxide film formed on a surface of an object such as a semiconductor wafer, particularly, to a technique for removing a native oxide film. Although the object to be processed in the present invention is not limited to a semiconductor wafer, in order to specifically describing a preferred embodiment of the present invention, the present invention in relation to a technique for removing a native oxide film formed in a bottom portion of a fine hole formed on a surface region of a semiconductor wafer is described.

FIG. 1 shows the construction of a processing apparatus 12 of the present invention, and FIG. 2 is a plan view showing a supply section of an NF₃ gas. As shown in the drawings, the processing apparatus 12 comprises a microwave generating source 58, waveguides 60, 62, a gas introducing section 44 for introducing a mixed gas of an N₂ gas and H₂ gas, a pipe 14 for forming a plasma and active species, and a process chamber 16 in which a native oxide film formed on an object (semiconductor wafer) is removed. The microwave formed in the microwave generating source 58 is supplied through the waveguides 62, 60 and the pipe 14 for forming a plasma and active species. Within the pipe 14, the mixed gas of the N₂ gas and H₂ gas is converted into a plasma so as to form active species. The process chamber 16 is in the form of a cylinder made of, for example, aluminum. A susceptor 20 made of, for example, SiC is supported by supporting rods 18 made of, for example, quartz. Formed in a bottom portion of the process chamber 16 is an exhaust port 22, which is connected to an exhaust system 25 equipped with a vacuum pump 24, etc. for evacuating the process chamber 16. An opening 26 equal in size to the susceptor 20 is formed below the susceptor 20. A heat ray transmitting member 28 made of, for example, quartz is hermetically attached to close the opening 26 using a sealing member 30. The heat ray transmitting member 28 is shaped arcuate in cross section in a manner to bend downward in the central portion.

A heating means 32 is arranged below the heat ray transmitting member 28 such that the susceptor 20 is heated from the back side. The heating means 32 includes a plurality of heating lamps 36, e.g., halogen lamps, mounted on a rotatable plate 34. The heat emitted from the heating means is transmitted through the heat ray transmitting member 28 so as to be incident on the back surface of the susceptor. Incidentally, heating means other than the heating lamps 36 shown in the drawing can also be used in the present invention. For example, it is possible to mount an electric heater within the susceptor 20. Further, a semiconductor wafer transfer port 38 and a gate valve 40 are formed in a side wall of the process chamber 16.

The pipe 14 for forming a plasma and active species, which is made of an electrically insulating and microwave transmitting material such as quarts, is hermetically mounted to close an upper opening in a ceiling portion 16A of the process chamber 16 using a sealing member 42. Also, the pipe 14 extends through the waveguide 60. The gas introducing section 44 is mounted on the upper end of the pipe 14 such that a mixed gas of an N₂ gas and an H₂ gas is introduced through the gas introducing section 44 and the waveguide 60 into the pipe 14. The gas introducing section 44 includes an introducing nozzle 46, which is connected to a gas passageway 48. The gas passageway 48 is connected to a two-way branched tube, one of the branches being connected to an N₂ gas source 52 via a flow rate controller 50, e.g., mass flow controller, and the other branch being connected to an H₂ gas source 54 via another flow rate controller 50.

The microwave having a frequency of, for example, 2.45 GHz, which is generated in the microwave generating source 58, is supplied through the waveguide 62 having, for example, a rectangular cross section, and the other waveguide 60, e.g., an Evenson type microwave, into the pipe 14 for forming a plasma and active species. The mixed gas of the H₂ gas and N₂ gas is converted into a plasma within the pipe 14 under the action of the microwave so as to form active species of N* from N₂ and H* from H₂.

An NF₃ gas supply section 68 is formed in a lower portion of an outlet port 64 of the pipe 14 so as to supply an NF₃ gas into the active species. The NF₃ gas is supplied from an NF₃ gas source 80 into a ring-like shower head 70 made of quartz through a flow rate controller 78, e.g., mass flow controller, a gas passageway 76 and a communicating pipe 74. Then, the NF₃ gas is added through a gas holes 72 to the active species. As a result, the NF₃ itself is also activated by the active species.

A guide member 66 made of quartz and downwardly flared or shaped conical is mounted to the outlet port 64 of the pipe 14 so as to permit the activated gases to be supplied uniformly onto an object to be treated.

The processing method of the present invention is worked by using the processing apparatus of the construction described above. Specifically, an object W (silicon wafer) is introduced into the process chamber 16 through the gate valve 40, which is opened, and disposed on the susceptor 20. The contact hole 2 as shown in FIG. 6A is formed in advance in a surface region of the silicon wafer W. As described previously, a native oxide (SiO₂) film 10 is formed in a bottom of the contact hole 2. FIG. 3A shows in a magnified fashion the native oxide film 10 formed in the bottom of the contact hole 2.

After introduction of the wafer W, the gate valve 40 of the process chamber 16 is closed to make the process chamber hermetic. Under this condition, the process chamber is evacuated to establish a vacuum condition. Then, the N₂ gas and H₂ gas are introduced at predetermined flow rates from the N₂ gas source 52 and the H₂ gas source 54 into the pipe 14 for forming a plasma and active species through the plasma gas introducing section 44. Also, the microwave having a frequency of, for example, 2.45 GHz, which is generated in the microwave generating source 58, is introduced into the pipe 14 through the waveguides 62 and 60. Within the pipe 14, the N₂ gas and the H₂ gas are converted by the microwave into a plasma and, activated to form active species. Since the process chamber 16 is held vacuum, these active species are moved toward the outlet port 64 of the pipe 14.

Further, the NF₃ gas supplied from the NF₃ gas source 80 is added through the ring-like shower head 70 of the NF₃ gas supply section 68 to the active species of the mixed gas of the H₂ gas and N₂ gas, with the result that the NF₃ gas is also activated by the active species so as to form a mixture of these activated gases. The mixture is effectively guided uniformly by the guide member 66 toward the wafer W so as to be brought into contact with the wafer W. As a result, the active species react with the native oxide film on the wafer W to cause the native oxide film to be modified into a reaction film 82 in which Si, N, H, F and O are mixed together, as shown in FIG. 3B. The reaction film 82 also serves to prevent the silicon wafer W itself from being etched with the activated gases. During this treatment, the wafer W is not heated, and is put under room temperature to cause the activated gases to reach a bottom of the contact hole effectively.

For example, the flow rates of the H₂ gas, N₂ gas, and NF₃ gas may be set at 20 sccm, 30 sccm and 200 sccm, respectively. The process pressure may be set at 3 Torr, the plasma power may be set at 150W, and the process time may be set at 3 minutes.

As described previously, the downwardly flared guide member 66 is arranged above the susceptor 20, with the result that scattering of the activated gases is suppressed by the guide member 66. It follows that the activated gases flow effectively toward the wafer so as to perform reaction with the native oxide film formed on the wafer W.

After formation of the reaction film 82, the supply of the H₂ gas, N₂ gas and NF₃ is stopped. Also stopped is the operation of the microwave generating source 58. Then, the process chamber 16 is evacuated to discharge the gases remaining within the process chamber 16 to the outside. Under this condition, the susceptor 20 is heated from the back surface by the heating means 32 so as to elevate the temperature of the wafer W disposed on the susceptor 20 to a predetermined level, e.g., at least 100° C. By this heating, a molecule 84 containing Si, N, H, F and O is sublimated so as to be scattered. As a result, the native oxide film 10 is removed from the wafer W to expose the Si substrate to the outside at the bottom of the contact hole. The heat treatment for removing the native oxide film 10 is carried out under a pressure of 1 mTorr or less for about 2 minutes.

The mechanism of the native oxide film removal described above has not yet been clarified sufficiently. However, it is considered reasonable to understand that the activated gases of H₂ gas, N₂ gas and NF₃ gas react with the native oxide film (SiO₂) to form a large molecule containing Si, N. H, F and O. To be more specific, N, F and H are considered to be interposed between O and Si so as to be bonded each other, and the resultant product is considered to be readily sublimated under heat of at least 100° C. while maintaining a molecular form of N—F—H—O—Si.

The atoms on the surface of the wafer W thus treated were analyzed by a surface elemental analysis apparatus (XPS, i.e., X-ray Photoelectron Spectroscopy), with the result as shown in FIGS. 4A and 4B. FIG. 4A shows the analytical result of the surface of the semiconductor wafer before the treatment, with FIG. 4B showing the analytical result of the wafer after the treatment of the present invention. As shown in FIG. 4A, a peak of the intensity is recognized in the vicinity of 104 eV of the binding energy, which denotes the Si—O bond, in addition to another peak in the vicinity of 99 eV of the binding energy denoting the Si—Si bond before the process of the present invention. After the process of the present invention, however, a peak is not recognized in the vicinity of 104 eV of the binding energy, which denotes the Si—O bond, though another peak is recognized in the vicinity of 99 eV of the binding energy denoting the Si—Si bond, as shown in FIG. 4B. This clearly supports that the native oxide film (SiO₂) has been completely removed by the treatment of the present invention.

Various experiments have been conducted by changing the process pressure and temperature in sublimating the reaction film 82. It has been found that the lower limit of the process temperature has been found to be about 100° C. If the process temperature was lower than about 100° C., the reaction film 82 was not sufficiently sublimated.

If the native oxide film is removed by the conventional wet washing, the surface of the object after removal of the native oxide film is relatively roughened. However, the surface of the object after removal of the native oxide film by the method of the present invention is rendered flat, compared with the wet washing. It follows that the contact area between the wafer surface after removal of the native oxide film and the surface of the wiring fixed to the wafer surface is increased, leading to a stronger bonding between the two.

Additional experiments have been conducted in an attempt to confirm the effect produced by the present invention. In these experiment, used as samples were, silicon substrates each having a silicon oxide film formed thereon. A contact hole was formed in the silicon oxide film. Further, a doped polycrystalline silicon (polysilicon) wiring was buried in the contact hole for electrical connection to the silicon substrate. Before formation of the polysilicon film, the sample was washed with a washing solution as a pretreatment in order to remove mainly particles. The washing solution consisted of ammonia water, hydrogen peroxide solution, and pure water mixed at a ratio of 1:2:10. The washing treatment was carried out at 60° C. for 10 minutes. Further, each of the samples was washed with water and, then, dried, followed by dividing the samples into a group of comparative samples and another group of samples to which the method of the present invention was applied. These comparative samples were etched with an etching solution consisting of hydrofluoric acid and pure water mixed at a ratio of 1:99. The etching treatment was carried out at room temperature for 10 minutes. The other samples for the present invention were subjected to a dry treatment under the conditions described previously. FIG. 7 shows the experimental data in respect of the contact resistance relative to various contact sizes. The comparative samples are denoted by an SCI-DFH curve in FIG. 7, with the samples for the present invention being denoted by an SCI-DRY curve. The expression “SCI” represents the washing pretreatment which was commonly employed in both the comparative samples and the samples for the present invention. As apparent from FIG. 7, the samples treated by the method of the present invention were lower in the contact resistance than the comparative samples. In addition, the difference in the contact resistance between the comparative samples and the samples for the present invention was prominently increased with decrease in the contact size.

As described above, the processing method and apparatus of the present invention permit removing a native oxide film formed at a bottom of a fine contact hole without using a washing liquid as in the wet washing method. It should also be noted that an NH₃ gas is not used in the present invention, making it possible to disuse an exhaust gas processing apparatus for processing the NH₃ gas. It follows that the facility cost and the operating cost can be reduced in the present invention.

In the present invention, activated gases are formed from an N₂ gas, H₂ gas and NF₃ gas. A surface of an object is exposed to these activated gases to bring about a reaction between these activated gases and an oxide film to be removed, particularly a native oxide film formed on the surface of the object. As a result, the oxide film is converted into a reaction film, which is then sublimated by the heating of the object to a predetermined temperature. It should also be noted that, in the present invention, a mixed gas of an N₂ gas and H₂ gas is converted into a plasma and active species are formed. Further, the NF₃ gas is added to the active species so as to form activated gases of the N₂ gas, H₂ gas and NF₃ gas. Additional features of the present invention are as already described herein before.

It is of no difficulty for those skilled in the art to arrive at various modifications based on the embodiment of the present invention described above. Of course, these modifications fall within the technical scope of the present invention. In other words, the technical scope of the present invention is not limited to the typical embodiment described herein in detail. Needless to say, various modifications are conceivable within the technical scope defined in the appended claims.

For example, a plasma forming section 56 included in the processing apparatus of the present invention is equipped with the microwave generating source 58, the Evenson type waveguide 60, etc., as shown in FIG. 1. The plasma forming section 56 of the particular construction is efficient for forming a plasma of a mixed gas consisting of N₂ gas and H₂ gas. However, it is also possible to substitute the plasma forming section 56 constructed as shown in. FIG. 5 for the plasma forming section shown in FIG. 1. To be more specific, the plasma forming section 56 shown in FIG. 5 comprises a high frequency generating source 86 generating RF (Radio Frequency) of 13.56 MHz and a induction coil 88 wound about a part of the pipe 14 for forming plasma and active species. Further, a matching circuit 90 is connected between the induction coil 88 and the high frequency generating source 86 so as to achieve the impedance matching therebetween. In this case, a plasma is formed by an inductive coupling upon supply of an RF wave to the induction coil 88, with the result that the function and effect similar to those in the embodiment described previously are exhibited.

The flow rates of the various gases described previously are no more than examples and, thus, do not limit the technical scope of the present invention.

In the embodiment described previously, the flow rates of the H₂ gas, N₂ gas and NF₃ gas are set at 20 sccm, 30 sccm and 200 sccm, respectively. However, it is possible to double these flow rates or to set the flow rates at levels higher than double of these flow rates.

Also, in the embodiment described previously, the shower head 70 for supplying the NF₃ gas is mounted to the outlet port 64 of the pipe 14 for forming plasma and active species. However, the mounting position of the shower head 70 is not limited to the outlet port 64 noted above. To be more specific, the shower head 70 can be mounted anywhere as far as the mounting position is closer to the object than the waveguide 60. In this case, it is important to add the NF₃ gas to an atmosphere of active species converted from the plasma, not to the plasma of a mixed gas consisting of H₂ gas and N₂ gas. If NF₃ is present within a plasma, the NF₃ gas is excessively activated so as to severely etch the wall made of quarts, i.e., the wall of the pipe 14 for forming plasma and active species.

Further, it is not absolutely necessary for the shower head 70 to have a ring-like structure. For example, pipes having gas holes may be arranged in a lattice shape to form the shower head. Alternatively, a simple gas introducing nozzle can be used in place of the shower head 70.

Further, the embodiment described previously is directed to removal of a native oxide film formed on a silicon substrate. Of course, the present invention can also be employed for removing a native oxide film formed on other substrates or layers. For example, the technique of the present invention can be employed for removing a native oxide film formed on a metal silicide film, e.g., film of WSi_(x), TiSi_(x), CoSi_(x), AlSi_(x) or NiSi_(x).

Further, the frequencies of the microwave and RF wave are not limited to those referred to previously. In other words, the microwave and RF wave of different frequencies can also be used in the present invention.

Further, in the embodiment described previously, a native oxide film was removed within the process chamber 16. Of course, it is necessary for at least an oxide film or a native oxide film to be removed within the process chamber 16. However, it is possible to carry out other treatments such as a film-forming treatment and an etching treatment within the process chamber 16 before or after removal of the native oxide film.

Further, in the embodiment described previously, the activated gas flows downward toward a surface of an object. However, it is not absolutely necessary for the activated gas to flow downward in the present invention. For example, it is possible to allow the activated gas to flow laterally, obliquely or upwardly toward a surface of the object.

Still further, in the embodiment described previously, heating lamps arranged below the back surface of a susceptor were used as a heating means for heating an object, which was disposed on the front surface of the susceptor. Alternatively, an electric heater arranged within the susceptor may be used as the heating means. It is also possible to arrange both an electric heater and a cooling means within the susceptor. The cooling means can be used for cooling the susceptor and the object in order to permit the activated gas to be moved to reach a bottom portion of a fine hole such as a contact hole formed in a surface region of the object. Further, heating lamps arranged above the object can be used for directly heating the surface of the object.

It is of no difficulty for those skilled in the art to arrive at various modifications in respect of the features of the present invention described above. Of course, these modifications fall within the technical scope of the present invention. In other words, the technical scope of the present invention is not limited to the typical embodiment described herein in detail. Needless to say, various modifications are conceivable within the technical scope defined in the appended claims.

FIG. 8 is a conceptual block diagram of a surface treatment apparatus 1 according to another embodiment of the present invention. The apparatus 1 can be used for removing a contaminant having a thickness of 10 Å to 20 Å, which is caused on the surface of a subject to be treated. The contaminant is, for example, a native oxide film or a chemical oxide naturally adhered to or formed on the surface of the subject, and referred to as a native oxide film hereinafter. The subject to be treated is, for example, a semiconductor wafer and referred to as a wafer W hereinafter. As shown in FIG. 8, the surface treatment apparatus 1 comprises a plasma generation pipe 130 for generating plasma from a mixture of N₂ and H₂ gases and activating the plasma, a treatment vessel 110 for removing the native oxide film from the surface of the wafer W, and a reactive-gas supply pipe 126 for supplying an NF₃ gas (reactive gas) from an NF₃-gas source into the treatment vessel 110.

The treatment vessel 110 is constituted of aluminum materials and its inner wall is provided with quartz (SiO₂) linings 113 and 114 to be protected from metal pollution, erosion or the like. The treatment vessel 110 can be formed as a housing whose transverse plane may have various shapes such as a circle, a square and a polygon. A bottom plate 112 having a predetermined thickness is fixed to the bottom of the treatment vessel 110. A base 129 is disposed on the bottom plate 112, and a cylindrical susceptor 120 is provided on the base 129. The wafer W is placed on the top of the susceptor 120 and clamped by a quartz-made clamp ring 121. A jacket (or a pipe) 122 for holding a chiller and a heat exchanger 123 are included in the susceptor 120. The jacket 122 and heat exchanger 123 can be formed integrally as one component. The chiller is supplied from a chiller supply unit 142 into the jacket 122 through a cooling pipe 143 to cool the wafer W down to a given temperature, such as a temperature not higher than room temperature.

The susceptor 120 is provided with a wafer-lifting means. The wafer-lifting means is a mechanism for lifting the wafer W up to a given heating position (L2) from the susceptor 120 to heat the wafer W, and, after predetermined treatment, lowering the wafer W and returning it on the susceptor 120 and includes a pin-driving mechanism 125, supporting pins 124 a, and arms 124. An example of the wafer-lifting means is illustrated in FIGS. 9A and 9B. The pin-driving mechanism (hydraulic cylinder) 125 is disposed under the base 129 formed at the bottom of the treatment vessel 110, and a horseshoe supporting piece 124 b is fixed to the end portion of a cylinder rod 125 a of the cylinder 125. The arms 124 extend inward along the radial direction of the wafer W, and the supporting pins 124 a are fixed to their respective points (three points) of the arms 124. The supporting pins 124 a each have an upward-protruding peak portion at one end, and the wafer W is thus held horizontally by these three peak portions of the supporting pins 124 a. To heat the wafer W using a heat radiation means (a heating lamp) 119, it is lifted up to the heating position (L2) shown in FIG. 8.

Exhaust pipes (e.g., four exhaust pipes) 140 are arranged on the periphery of the bottom plate 112 fixed to the bottom of the treatment vessel 110. An exhaust means (e.g., a vacuum pump) 141 is connected to these exhaust pipes 140 to evacuate the treatment vessel 110 to produce vacuum.

A top plate 111 (made of, e.g., aluminum materials) is fixed to the top of the treatment vessel 110. A quartz-made cover (dome) 115 having a flange section 116 is formed on the top plate 111, with a sealing member (e.g., a rubber-made O-shaped ring) 117 interposed therebetween. The cover 115 can be formed integrally with the quartz-made plasma generation pipe 130 as one component and may have various shapes such as a flat one and a domed one. Monitor devices such as a pressure sensor can be provided at a sealing section including the sealing member 117. These monitor devices monitor a sealing pressure of the sealing section, a gas leak from the sealing section, and the like.

A number of heating lamps 119 are provided above the cover 115 as a heat radiation means for heating the wafer W from above. As the heating lamps 119, halogen lamps can be employed such that the wafer W can be heated quickly. The heat waves radiated from the heating lamps 119 enter the surface of the wafer W lifted up to the heating position, through the transparent quartz-made dome 115, thereby heating the wafer W up to a temperature of 100° C. or higher (e.g., 120° C.).

Since the heating lamps 119 are covered with a cover 118 made of metal or the like, they can be prevented from radiating heat waves and light rays to the outside and, even when the quartz-made dome 115 is broken, plasma gas or reactive gas can be prevented from diffusing and leaking to the outside.

A gate valve 110 a is provided on the side wall of the treatment vessel 110 to communicate with a carrier chamber, a load-lock chamber, and the like. The gate valve 110 a is opened and closed when the wafer W is carried in and out.

Usually the-inner surface of the gate valve 110 a need not be protected by quartz because the metal surface of the treatment vessel 110 is hardly etched by NF₃ gas. Coating of the metal surface with quartz is generally performed in order that the metal surface can prevent the lifetime of species activated by plasma from being shortened. In this sense, it is favorable to coat the inner surface of the gate valve 110 a with quartz, too.

The quartz-made plasma generation pipe 130 can be provided in the upper central part of the quartz-made cover 115 integrally with each other by melting contact, and it is opened to the treatment vessel 110 in the center of the cover 115 to introduce plasma into the vessel 110. For generation and introduction of plasma, any structure capable of uniform surface treatment can be adopted. For example, plasma can be introduced from an opening shifted from the center of the cover 115 or from the side of the treatment vessel 110.

An introduction section 133 for introducing a plasma generating gas is connected to the upper end portion of the plasma generation pipe 130. Both N₂ and H₂ gases are supplied from N₂ and H₂ gas sources 135 and 136 to a gas passage 133 a through a flow-rate controller (MFC) 134. A mixture (N₂+H₂) of these gases is supplied to a plasma generation section of the plasma generation pipe 130 inside a plasma cavity 131 through the introduction section 133.

A microwave generator 132 is connected to the plasma cavity 131. A microwave of, e.g., 2.45 GHz, which is generated from the microwave generator 132, is applied to the plasma cavity 131 to excite the plasma generating gas in the plasma generation pipe 130, activate the mixture of N₂ and H₂ gases, and form active gas species of N* and H* radicals. These active gas species are introduced into the treatment vessel 110 from an opening 130 a of the plasma generation pipe 130.

A number of gas exhaust nozzles 126 a for supplying an NF₃ gas are arranged in position L₁ under the opening 130 a of the plasma generation pipe 130. The position L1 is at least 20 cm or more, preferably 30 cm or more away from the lower end of the plasma cavity 131 of the plasma generation pipe 130 (plasma generation section). A given amount of NF₃ gas is supplied to the gas exhaust nozzles 126 a through an NF₃-gas source 128, a flow-rate controller (MFC) 127, a conduction pipe 126, a pipe 126 b surrounding the outer wall of the treatment vessel 110, and a conduction pipe 126 c penetrating the wall of the vessel 110.

The gas exhaust nozzles 126 a shown in FIG. 8 are protruded slightly inward from the inner wall of the treatment vessel 110. However, neither the gas exhaust nozzles 126 a nor the gas supply section from the conduction pipe 126 to the gas exhaust nozzles 126 a is limited to the structure shown in FIG. 8. Another gas supply section is illustrated in FIG. 15. Referring to FIG. 15, a pipe 126 b and conduction pipes 126 c are provided in the treatment vessel 110 made of aluminum. These pipes 126 b and 126 c can be formed integrally with the inner wall of the treatment vessel 110. Gas exhaust nozzles 126 a are provided so as not protrude from the inner wall of the vessel 110, which allows gas to be diffused uniformly in the treatment vessel 110 and does not disturb a flow of plasma from the upper reaches.

A shower head 261 b is illustrated in FIG. 10A as an alternative to the gas exhaust nozzles 126 a shown in FIG. 8. The shower head 261 b is shaped like a ring and made of quartz. The shower head 261 b has a number of gas exhaust nozzles 261 a. These nozzles 261 a are arranged on the circumference of the shower head 261 b so as to be directed in the downward direction (in the direction of the susceptor 120), the lateral direction, or the slanting direction. A conduction pipe 261 is connected to the ring-shaped shower head 261 a. The shower head 261 a is located horizontally in a given position within the treatment vessel 110 to supply NF₃ gas into the treatment vessel 110.

FIG. 10B shows a grid-like shower head 262 b having a number of gas exhaust nozzles 262 a. In this shower head 262 b, too, the gas exhaust nozzles 262 a can be arranged in the downward direction, lateral direction, or the slanting direction.

Furthermore, a means (not shown) for regulating the flow of plasma gas can be provided at the opening 130 a of the plasma generation tube 130. The regulating means can be formed as a cylindrical or umbrella cover which is opened from the opening 130 a toward the susceptor 120.

A method of treating a surface (removing a native oxide film) by the foregoing surface treatment apparatus, will now be described with reference to the flowchart of FIG. 11.

STEP (a):

When the gate valve 110 a of the surface treatment apparatus 1 shown in FIG. 8 is opened, a single wafer W is carried from the carrier chamber into the treatment vessel 110 through the gate valve 110 a in an unreactive atmosphere (e.g., in a vacuum), placed on the susceptor 120, and clamped to the susceptor 120 by the clamp ring 121. In the preceding step, a contact hole 2 is formed in the wafer W as shown in FIG. 6A, and an oxide 10 such as a native oxide film is formed at the bottom of the contact hole 2 as shown in FIG. 6A.

STEP (b):

After the wafer W is inserted into the treatment vessel 110, the gate valve 110 a is closed. The vessel 110 is evacuated through the exhaust pipes 140 by means of the vacuum pump 141 to produce a vacuum atmosphere of 1 mTorr or lower (133 Pa or lower).

STEP (c):

The wafer W is cooled to room temperature or lower by the chiller supplied from the chiller supply unit 142 to the susceptor 120.

STEP (d):

N₂ (nitrogen) and H₂ (hydrogen) gases are supplied from the N-gas and H-gas sources 135 and 136 to the gas passage 133 a, under the control of their flow rates through the flow-rate controller (MFC) 134, to generate a mixture gas (N₂+H₂), and the mixture gas is then supplied from the plasma gas introduction section 133 to the plasma generation pipe 130 as a plasma generating gas.

STEP (e):

A microwave (2.54 GHz) is generated from the microwave generator 132 and introduced into the cavity formed around the plasma generation section of the plasma generation pipe 130. The microwave causes plasma to be produced from the mixture gas (N₂+H₂). The plasma is activated to form active gas species of N* and H* radicals. In particular, the H₂ gas, which is originally hard to become plasmatic, can be changed into plasma with efficiency and then activated, together with the N₂ gas. The active gas species N* and H* are attracted to the vacuum atmosphere in the treatment vessel 110 and caused to flow from the plasma generating section of the pipe 130 toward the opening (outlet) 130 a thereof.

STEP (f):

A reactive gas NF₃ is supplied from the NF₃-gas source 128, provided outside the treatment vessel 110, to the reactive-gas conduction pipe 126 through the flow-rate controller (MFC) 127 and then supplied from the gas exhaust holes 126 a into the treatment vessel 110 like a shower. The NF₃ gas is added to the active gas species of N* and H* radicals generated by the plasma of N₂ and H₂ gases flowing from the opening 130 a of the plasma generation pipe 130. The added NF₃ gas is thus activated by these active gas species N* and H*.

STEP (g):

By the activation of NF₃ gas and the synergistic effect of active gas species N* and H*, the native oxide film 10 of the wafer W shown in FIG. 3A is degenerated into a reactive film 82 mixing Si, N, H, F and O, as shown in FIG. 3B. When the native oxide film 10 is degenerated, a chiller (e.g., ethylene glycol) is supplied into the susceptor 120 by the chiller supply unit 142, and the wafer W placed on the susceptor 120 is cooled to not higher than room temperature.

This cooling increases the rate of etching using the NF₃ active gas. As process conditions for the above treatment, it is preferable that the flow rates of H₂, NF₃ and N₂ be 10 sccm, 130 sccm and 100 sccm, respectively, process pressure be 3 Torr, plasma power be 50W, and process time be about 3 minutes.

Since the etching rate of etching species generated by reaction of NF₃ and H₂ gases is low, these gases are adsorbed by the etching surface to determine the etching rate. If the wafer is cooled down to not higher than room temperature, the above adsorbing rate is increased and so is the etching rate.

As described above, it is preferable that the step of degenerating the native oxide film 10 on the wafer W by the activation of NF₃ gas and the synergistic effect of active gas species N* and H* be executed at a temperature which is not higher than room temperature. FIG. 16 shows a relationship between cooling temperature and etching rate in the degenerating step with respect to six cases. In this figure, the ordinate indicates the etching rate, while the abscissa does the cooling temperature of the wafer W at the start of the step. As is apparent from FIG. 16, since the control characteristic of etching becomes unstable when the cooling temperature exceeds 20° C. it is preferable that the cooling temperature be not higher than room temperature and range from 20° C. to −20° C. and from 10° C. to −20° C. The data shown in FIG. 16 is obtained under the following experimental conditions: In the step of degenerating the native oxide film 10, ratio of H₂/NF₃/N₂ is 300:60:400 sccm, pressure is 4 Torr, power is 300W, and process time is 1 minute. In the sublimation step, temperature is 140° C., time is 1 minute, and atmosphere is vacuum.

STEP (h):

After the native oxide film is degenerated, the supply of H₂, N₂ and NF₃ gases is stopped, as is the driving of the microwave generator 132, thereby stopping the introduction of microwaves into the plasma generation pipe 130. The treatment vessel 110 is evacuated through the exhaust pipes 140 to produce vacuum.

STEP (i):

The wafer-lifting means is driven to lift the wafer W up to the heating position at least 5 mm away from the susceptor 120.

STEP (j):

When the heating lamp 119 lights up, the wafer W is heated from above and its surface is quickly heated to a temperature of 100° C. or higher (e.g., 120° C.) from room temperature.

STEP (k):

The reactive film 82 mixing Si, N, H, F and O is sublimated as gas 84 mixing Si, N, H, F and O by the heating lamp 119, as shown in FIG. 3C, and then eliminated and exhausted from the exhaust pipes 140. By this sublimation, the native oxide film 10 (reactive film 82) is removed from the wafer W and thus silicon (Si) appears on the surface of the wafer W. As the process conditions for this step, it is preferable that process pressure is 1 mTorr or lower and process time is about 2 minutes.

STEP (l):

The heating lamp 119 is extinguished.

STEP (m):

Finally, the evacuation is stopped.

STEP (n):

The gate valve 110 a is opened, and the wafer-lifting means is driven to lower the wafer W and return it on the susceptor 20. The wafer W from which the native oxide film is removed, is taken out of the treatment vessel 110 and carried to its next chamber (e.g., a carrier chamber) in a vacuum atmosphere.

The above oxide film includes not only SiO₂ but also W, Ti, Al, Ni and Co and a very thin oxide film (10 Å to 20 Å) grown on silicide of these elements.

The surface treatment apparatus of the present invention constitutes a multi-chamber cluster system in combination with other treatment units such as a metal-wiring forming chamber, a heating chamber, a cooling chamber, a carrier chamber and a load-lock chamber. The constitution of the cluster system will now be described.

FIG. 12 shows a vacuum cluster system 300 capable of carrying a wafer in an unreactive atmosphere (e.g., a vacuum atmosphere). The system 300 includes a native oxide film removing chamber 301 which corresponds to the surface treatment apparatus of the present invention. A heating chamber 302, at least one metal-wiring forming chamber 303, and a load-lock chamber 304 are connected to the chamber 301 through a carrier chamber 305. The metal-wiring forming chamber 303 forms metal wirings of Al, Ti, TiN, Si, W, WN, Cu, Ta and SiN on a subject to be treated by metal CVD. A gate valve 307 is interposed between adjacent chambers, and a carrier robot 306 is provided in the carrier chamber 305.

A wafer cassette containing wafers is transported into the load-lock chamber 304, and the wafers are carried in the carrier chamber 305 and aligned with reference to their orientation flats. When the gate valve 307 is opened, the carrier robot 306 carries the wafers into the native oxide film removing chamber 301 one by one. In the chamber 301, an oxide film is removed from the surface of each of the wafers, and the wafers are pre-heated in the heating chamber 302. After that, in the metal-wiring forming chamber 303, metal wirings such as Al and Ti are formed in the contact holes of the wafers by metal CVD. Finally, the wafers are returned to the load-lock chamber 304.

FIG. 13 illustrates a vacuum cluster system 200 capable of carrying a wafer in an unreactive atmosphere. The system 200 includes a native oxide film removing chamber 201 corresponding to the surface treatment apparatus of the present invention. A heating chamber 202, at least one metal-wiring forming chamber 203, a cooling chamber 204, and a load-lock chamber 205 are connected to the chamber 201 through a carrier chamber 206. A gate valve 208 is interposed between adjacent chambers, and a carrier robot 207 is provided in the carrier chamber 206.

In order to carry the wafers on which metal wirings are formed, from the chamber 203 which is usually heated to a temperature of about 500° C., to the load-lock chamber 205, the wafers need to be cooled to a temperature (about 150° C.) at which the chamber 205 can receive the wafers.

In the vacuum cluster systems shown in FIGS. 12 and 13, if the heating chambers 302 and 202 have a means for heating the wafers to a temperature of 100° C. or higher, the heating means of the native oxide film removing chambers 301 and 201 can be excluded.

The vacuum cluster systems so constituted have the advantages that a native oxide film can be prevented from being regenerated while the wafers are being carried in the atmosphere, time from removal of a native oxide film to deposition need not be controlled, a water mark can be prevented from being produced, a native oxide film can be removed from the wafer in situ, and throughput is greatly improved.

In the above-described surface treatment method and surface treatment apparatus according to the present invention, gas containing N and H is activated as plasma gas to form an active gas species. The active gas species activates a reactive gas (NF₃ gas) and cools a subject to be treated to not higher than room temperature. These three gases are reacted with an oxide film formed on the surface of the subject to degenerate the oxide film into a reactive film. If the reactive film is sublimated by heating, the oxide film such as a native oxide film can be removed from the surface of the subject with high efficiency and at a high etching rate.

In the foregoing embodiment, N₂ and H₂ gases are employed as a gas containing N and H. This gas can be replaced with another gas such as ammonium.

Furthermore, the shower head for jetting an activated gas is shaped like a ring or a grid. However, any other structure can be adopted for the shower head.

The surface treatment apparatus of the present invention and other treatment apparatuses constitute a cluster system capable of carrying a subject to be treated in an unreactive atmosphere or in a vacuum. Therefore, no oxide film is formed on a wafer while the wafer is being carried, and the system throughput is enhanced as a whole.

Additional advantages and modifications will readily occurs to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

What is claimed is:
 1. A surface treatment method comprising the steps of: carrying a subject to be treated, which has an oxide on a surface thereof, into a treatment vessel; introducing a gas mixture containing N and H elements into a plasma generation section, generating plasma from the gas, and activating the plasma to form an activated gas species of N and H elements; causing the activated gas species to flow toward the subject and adding an NF₃ gas to the activated gas species to generate an activated gas of the NF₃ gas; reacting the activated gas of the NF₃ gas with the oxide on the surface of the subject cooled to not higher than a predetermined temperature to modify the oxide into a reaction film.
 2. The surface treatment method according to claim 1, wherein the gas containing N and H elements is a mixture gas of N₂ and H₂ gases, and the method further comprises the steps of stopping supply of N₂, H₂ and NF₃ gases into the treatment vessel and heating the subject to a predetermined temperature to sublimate the reaction film, after the step of modifying the oxide into the reaction film; and taking the subject out of the treatment vessel.
 3. The surface treatment method according to claim 2, wherein the predetermined temperature at which the reaction film is sublimated, is not lower than 100° C.
 4. The surface treatment method according to claim 1, wherein the predetermined temperature at which the subject is cooled, is not higher than room temperature.
 5. The surface treatment method according to claim 1, wherein the predetermined temperature at which the subject is cooled, ranges from 20° C. to −20° C.
 6. The surface treatment method according to claim 1, wherein the predetermined temperature at which the subject is cooled, ranges from 10° C. to −20° C.
 7. A method of removing an oxide film formed on a surface of an object to be processed, comprising the steps of: forming an activated gas from a N₂ gas, H₂ gas and NF₃ gas, said forming step comprising: forming a plasma of a mixed gas consisting essentially of the N₂ gas and H₂ gas in a plasma generating section, and also forming active species; supplying the NF₃ gas into said active species in a section following the plasma generating section so as to form said activated gas of the N₂ gas, H₂ gas and NF₃ gas; exposing the surface of said object to said activated gas to bring about a reaction between the activated gas and the oxide film formed on the surface of the object, thereby forming a reaction film; and heating the object to a predetermined temperature so as to sublimate said reaction film.
 8. The method according to claim 7, wherein the oxide film to be removed is a native oxide film formed by the reaction with the air atmosphere during predetermined process steps applied to said object.
 9. The method according to claim 7, wherein the formation of the plasma of the mixed gas consisting essentially of the N₂ gas and H₂ gas is carried out in the plasma generating section by introducing a microwave into the mixed gas of the N₂ gas and H₂ gas supplied to the plasma generating section.
 10. The method according to claim 7, wherein said predetermined temperature is at least 100° C., and each of said reaction film and said activated gas is formed at a temperature lower than 100° C.
 11. A surface treatment method comprising the steps of: carrying a subject to be treated, which has an oxide on a surface thereof, into a treatment vessel; introducing a gas mixture consisting essentially of N and H elements into a plasma generation section, generating plasma from the gas, and activating the plasma to form an activated gas species of N and H elements; causing the activated gas species to flow toward the subject and adding an NF₃ gas to the activated gas species to generate an activated gas of the NF₃ gas; reacting the activated gas of the NF₃ gas with the oxide on the surface of the subject; and taking the subject out of the treatment vessel.
 12. A surface treatment method comprising the steps of: carrying a subject to be treated, which has an oxide on a surface thereof, into a treatment vessel; introducing a gas mixture consisting essentially of N and H elements into a plasma generation section, generating plasma from the gas, and activating the plasma to form an activated gas species of N and H elements; causing the activated gas species to flow toward the subject and adding an NF₃ gas to the activated gas species to generate an activated gas of the NF₃ gas; reacting the activated gas of the NF₃ gas with the oxide on the surface of the subject cooled to not higher than a predetermined temperature; and taking the subject out of the treatment vessel.
 13. A surface treatment method comprising the steps of: carrying a subject to be treated, which has an oxide on a surface thereof, into a treatment vessel; introducing a gas mixture consisting essentially of N and H elements into a plasma generation section, generating plasma from the gas, and activating the plasma to form an activated gas species of N and H elements; causing the activated gas species to flow toward the subject and adding an NF₃ gas to the activated gas species to generate an activated gas of the NF₃ gas; reacting the activated gas of the NF₃ gas with the oxide on the surface of the subject to modify the oxide into a reaction film; and taking the subject out of the treatment vessel. 